A prior art overcasting attachment for use with a sewing machine is disclosed in U.S. Pat. No. 4,546,715 to the inventor of the present invention, in which the vertical movement of a needle bar of the straight stitch sewing machine is transmitted to a swing lever to be converted into a swinging movement in a horizontal plane, and the movement of the swing lever causes an upper cross thread guide lever and a lower cross thread guide lever to move relatively longitudinally toward and away from a needle through pins movable along corresponding profiled cam slots formed in the swing lever, so that a predetermined thread-engaging operation may be performed to the needle thread and bobbin thread for straight stitching. ln forming overcasting stitches, the above process further includes a step of engaging an upper cross thread with a lower cross thread. According to the above patent, while the upper cross thread guide lever is moved substantially in a horizontal plane, the lower cross thread guide lever is swung from up to down to once engage the lower cross thread with the upper cross thread guide lever, and then the upper cross thread guide lever is moved ahead of the lower cross thread guide lever to complete the overcasting stitches. Therefore, the lower cross thread guide lever is required to be swung laterally as well as to be moved vertically at the distal end thereof, and such vertical movement is effected by providing a guide member having a vertical guide slot and by engaging a pin provided at the distal end of the lower cross thread guide lever with the guide slot.
However, in such a construction, the profiled slots and the guide slot are used to impart the predetermined movements on the upper cross thread guide lever and the lower cross thread guide lever. This tends to increase vibration and noise to be generated during movement of the pins in the slots as well as complexity of the construction, so that improvement in this field has been desired.